Paper manufacture, in the simplest sense, involves producing a pulp from wood, slurrying the pulp and water, and forming a pulp mat, which is pressed and dried to form paper. In the critical forming step, the pulp/water slurry (furnish) is formed as a mat on the wire web of the paper machine. Excess water and fines (white water) pass through the mat on the wire and are recycled. The formed web goes forward into the press and dryer section of the machines where the mat becomes paper.
Broke paper is the term used in the paper industry to describe the paper which does not meet specifications and for that reason cannot be sold. This paper is usually recycled internally at the mill to recover fibers but it may also be sold to other mills as a source of fiber. The broke paper may be coated, the coating being applied to the base sheet of paper as it is manufactured. The broke paper which is coated is referred to as coated broke paper. Waste paper is the term used in the paper industry to describe paper which has been utilized by a consumer. It is often termed "post consumer waste." This paper is often collected and recycled at a mill to recover fibers. The waste paper may be coated, the coating being applied to the base sheet of paper as it is manufactured. The waste paper which is coated is referred to as coated waste paper. Coated paper that is recycled can be broke or waste paper. In recent years many paper mills have experienced problems with the recycling of coated paper because the coatings introduce materials which normally would not be present in the original stock of fibers used to manufacture the base paper sheet.
The coatings normally comprise various pigments and binders. Typical pigments used include many types of clay, calcium carbonate, titanium dioxide, and other specialty fillers. The problems of white pitch are thought to be mainly caused by the binders which include latex polymers derived from styrene-butadiene and polyvinyl acetate resins and natural binders such as starch.
White pitch problems have been known for some time in the paper producing industry. White pitch is sticky, light gray substance which is found as a deposit on metal surfaces in the in the wet-end, forming press, or dryer sections of the paper machine. It is termed "white" to distinguish it from the brown or black pitch, which results from materials contained in the wood. White pitch is also found in the white water system. At times the pitch deposits carbonize to give black deposits in the dryer section of the paper machine. The white pitch problem has been shown to be caused by the relatively high use of coated paper in the furnish of mills experiencing the problem. When coated paper is re-pulped, the clay or minerals and the latex in the coatings do not readily disperse into the pulp but form agglomerations, which result in white pitch. White pitch can coat the equipment or form defects in the paper if it travels into the paper machine with the pulp. High machine downtime, frequent cleaning, paper sheet defects such as holes, and increased number of sheet breaks are costly problems associated with white pitch deposits. Equipment clean up is quite involved because deposits can be found on the foils, table rolls, vacuum boxes, dryer cans and dryer felts, and throughout the press felts.
Various solutions have been suggested for dealing with the white pitch problem. Several deposit control chemicals are currently being used or evaluated by the paper industry. By trapping and dispersing the small latex particles in the sheet, the white pitch problem can be controlled. More specifically, the latex particles should be attached to the fibers immediately passing through the re-pulper. At this point the latex particles are small and anionic, and therefore, they can exit the system as part of a sheet. Due to the anionic character of both the latex particles and the fibers, an additive having low molecular weight and high cationic charge is best suited for this purpose. However, the additive alone may not be sufficient to contain the latex particles in the paper sheet and the use of a retention aid compatible with the additive may be important for successful control of white pitch.
Synthetic polymers are the most successful known antideposition additives for white pitch. They are highly cationic, enabling them to create a strong electrostatic bond between the fibers, the latex particles and the additive. Once bonded, the fiber will carry the latex particles through the mill, with the help of a retention aid, and the particles will become part of the finished paper. Medium molecular weight polyglycol, amine/glycol or polyethyleneimine polymers have been have been shown to be useful in reducing white pitch.
Some of the methods for treating white pitch problems are described in documents below.
U.S. Pat. No. 5,131,982 (Michael R. St. John) describes the use of DADMAC containing polymers and copolymers to treat cellulose fibers recycled from coated broke recovery to make them suitable for making paper.
U.S. Pat. No. 4,997,523 (Pease et al) describes the use of a tetrafunctional alkoxylated diamine in combination with a phosphate compound, phosphonate compound or phosphoric acid to minimize the deposition of white pitch on paper making equipment.
U.S. Pat. No. 4,643,800 (Maloney et al) describe the use an oxyethylene glycol nonionic surfactant in which one end hydroxyl group has been substituted with an aliphatic or alkylaromatic group and the other end hydroxyl group has been replaced with a polyoxypropylene group or a benzyl ether group in combination with a medium molecular weight (500-50,000) polyelectrolyte dispersant to remove and disperse contaminants from secondary fiber during re-pulping.
There are several disadvantages of the use of polymers to control white pitch. Polymers are not generally cost efficient. For example, polyethyleneimine (PEI), a tertiary amine polymer, is an effective white pitch control additive yet it is quite costly to use. Also, as the length of a polymer increases the chance of the polymer breaking down and releasing the latex particles also increases. Polymers can even cause deposits due to the difficulty in controlling the polymer from attaching latex particles to other latex particles. Moreover, polymers are not generally water soluble which limits their applicability.
There are other solutions used for control of white pitch. Talc was commonly used in the past and is still sometimes used to control deposits. As a surface-active filler, talc acts to control deposits by drying the area around the pitch particle so that it cannot attach to the paper making equipment. However, this offer only a temporary solution to the pitch problem which reappears as the process continues. Talc does not bind the latex particles to the fibers, and therefore when exposed to shear, new tacky areas appear causing deposits. Also, additives which react with the surface of the pitch particle to render it less tacky (detackifiers) offer temporary solution to controlling white pitch. As with talc, this solution fails when exposed to shear. Both talc and tackifiers can be effective only when used with a good retention aid. The objective of retention aids is to cause latex particles to repel each other which sends them through white water. Eventually, pitch is deposited on the pump impellers, tank walls, and agitators.
It is evident that there is a need in paper making industry for an improved method of controlling the white pitch problem experienced with the recycling of coated paper.
Surprisingly, it has now been found that the deposition of white pitch on the paper making equipment can be inhibited in a simple and effective manner by the addition of ethyleneamine compounds to the coated paper during the re-pulping operation.